Various types of oxygen sensors have been proposed; polarographic oxygen sensors operate in accordance with the well known principle that, if an ion conductor forming a solid electrolyte body has electrodes applied thereto across which a voltage is placed, the current in the so-formed circuit will be representative of the oxygen concentration in the gases to which the element is exposed. Reference is made, for example, to U.S. Pat. No. 3,691,023, Ruka, and to U.S. application Ser. No. 637,998, filed May 12, 1975, and now abandoned (and published as British Pat. No. 1,523,550, Canadian Pat. No. 1,071,709, German Disclosure Document No. 26 54 483). The sensor of the aforementioned application Ser. No. 637,998 has a space in which the cathode electrode is placed which is separated from the gas to be sensed, and which is in communication with the gas to be sensed by an opening. By varying the size of the opening, diffusion of gas can be matched to the size of the electrode. If the opening is too large, the current which is measured in the circuit is practically independent of the actual concentration level of oxygen within the test gas, since the sensor will no longer operate in the diffusion limit current range. The space in which the cathode is placed is comparatively large, so that the effect will be that of exposing the cathode electrode to a surface of mixing chamber in which mixing is carried out slowly and with substantial inertia so that the response speed of sensor to changes in the oxygen level is slow. Such sensors, as described, are comparatively large and of complex manufacture.